In the past, various formulations have been employed as histological fixatives to provide microscopic detail to sections of biological tissue. One such commonly used fixative is that known as B-5, which is an aqueous solution of mercuric chloride, sodium acetate and formaldehyde. Disadvantages of this fixative include dangerous exposure of laboratory personnel to toxic mercury and formaldehyde and the unpleasant odor of the latter, expense of disposing of the fixative in a safe manner because of its mercury content, excessive wear on microtome blades due to the hardening of the tissue by mercury and the necessity of removing elemental mercury from tissue sections by treatment with iodine. Due to precipitation of calomel and metallic mercury upon standing, fresh solutions must be prepared daily and any excess solutions create disposal problems.
The various disadvantages associated with the presence of mercury preclude the use of B-5 fixative as a routine fixative and allow its use only in situations where microscopic detail is of the greatest significance and importance, for example in the classification of a malignancy diagnosed as a malignant lymphoma. Fortunately, the handling of tissue following fixation by B-5 is almost the same as that of tissue fixed in a routine fixative, such as neutral buffered formalin, except for the use of iodine.
While various other fixatives for general use have been proposed that are mercury and formaldehyde free these have generally required the use of special non-standard chemicals and present staining problems of one type or another that do not lend them to widespread use.